The term assassin is derived from the Arabic Hashshashin, describing medieval caravan raiders based in Alamut. See that article for details of the group.

In its most common use, assassin has come to mean someone who kills (assassinates) people selectively, usually for political reasons. The immediate motivation of an assassin may be money (in the case of a hit man), personal belief, orders from a government, or loyalty to a leader or group. Assassins are distinguished from snipers, or other soldiers who may employ the same methods, in that the latter are engaged in declared war between nation-states. The distinction blurs when a sniper, soldier, or spy is given a specific target, or if the orders come through unofficial channels. Terms such as death squad came into use to describe such unofficial killing.

The definition of an "assassin", as with "spy" or "terrorist", is politically loaded, and most commentators do not believe it has an objective definition.

Entire organizations have sometimes specialized in assassination as one of their services. Besides the original Hashishim, the ninja clans of Japan were rumored to perform assassinations. In the United States, Murder Incorporated, an organization with ties to the Mafia, was formed for the sole purpose of performing assassinations for organized crime.

It was deemed at that time that the liability of engaging in this activity led in general to a reduced level of personal security for elected leaders of democratic countries, who are in general much more vulnerable to retaliation. President Ford himself had been the target of two assassination attempts within 18 days of each other in 1975, although the attempted assassins' motives were not deemed to be financial or political. The still-controversial assassination of President John F. Kennedy thirteen years earlier in 1963 may also have been a factor in President Ford's executive order.

For democratic nation-states to claim to be better rulers than their less democratic opponents, they could not seem to be employing any assassin against leaders of political movements--thus acknowledging inability to compete with their leadership ideologically--a fatal weakness for any democratic government.

The public pose of democratic governments in general, with the notable exception of the state of Israel, was to disdain "trial, conviction, and death by intelligence." (Anonymous US military officer).

Also, as CIA spokesman Bill Harlow asserted in 2001, "The CIA has never turned down a field request to recruit an asset in a terrorist organization." Such groups are known to execute people in custody, attack civilians, and employ banned weapons, raising the issue of whether the CIA or other nation-state military intelligence agencies recruiting them are morally liable for these actions, especially if they are committed after recruitment.

However, during the 2001 Afghanistan War, local troops equipped, fed, and in some cases paid by the United States executed prisoners in their custody -- without sanction -- raising the question of moral and legal liability for this.

Some questioned whether the United States had avoided employing its own troops simply to avoid taking casualties -- and over-exposing its opponents, the Afghan Taliban, to atrocities from its Afghan Northern Alliance allies, their bitter enemies. The issue in general got little attention.

Patricia Zengel, in "Assassination and the Law of Armed Conflict", 1991, is summarized by Calder as concluding "...that there is no longer any convincing justification for retaining a unique rule of international law that treats assassination apart from other uses of force."

This conclusion is controversial, obviously, and rarely stated in public. The debate on the definition and use of the term "assassin" is inseparable from the similar debates surrounding freedom fighter, terrorist, guerilla, spy, saboteur, provocateur, double agent and other terms which are commonly used to describe players in asymmetric warfare. It is only seemingly neutral when no loyalty or political motive is claimed or assumed, and only money motivates.